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Synchromesh Gear Box

This type of gearbox is similar to the constant mesh type in that all the gears on the main shaft are in constant mesh with the corresponding gears on the layshaft. The gears on the layshaft are fixed to it while those on the main shaft are free to rotate on the same. Its working is also similar to the constant mesh type, but in the former, there is one definite improvement over the latter. This is the provision of a synchromesh device which avoids the necessity of double-declutching. The parts which ultimately are to be engaged are first brought into frictional contact which equalizes their speed, after which these may be engaged smoothly.

Reason of synchromesh gearbox preferred over constant mesh gearbox

1) No need of double-declutching as in case of constant mesh gearbox.
2) Smooth engagement of higher gears due to the synchromesh device.
3) Less noisy as helical gears are used.
4) Less vibration.

The figure shows the construction and working of a synchromesh gearbox. In most of the cars, however, the synchromesh devices are mot fitted to all the gears as is shown in this figure. They are fitted only on the high gears and on the low and reverse gears ordinary dog clutches are only provided. This is done to reduce the cost.

In the above figure is the engine shaft, Gears B, C, D, E are free on the main shaft and are always in mesh with corresponding gears in the layshaft. Thus all the gears on the main shaft as well as on layshaft continue to rotate so long as shaft A is rotating. Menders F1 and F2 are free to slide on splines on the main shaft. G1 and G2 are ring-shaped members having internal teeth fit onto the external teeth members F1 and F2 respectively. K1 and K2 are dog teeth on B and D respectively and these also fit onto the teeth of G1 and G2. S1and S2 are the forks. T1and T2 is the ball supported by springs. These tend to prevent the sliding of members G1 (G2) on F1 (F2).however when the force applied in G1 (G2)

through fork S1 (S2) exceeds a certain value, the balls are overcome and member G1 (G2) slides over F1

(F2). There are usually six of these balls symmetrically placed circumferentially in one synchromesh device.
M1, M2,
N1, N2,
P1, P2,
R1, R2
are the frictional surfaces.

The working of the gearbox is as follows .for direct gear, member G1 and hence member F1 (through spring-loaded balls) is slid towards left till comes M1 and M2 rub and friction makes their speed equal. Further pushing the member G1
to left causes it to override the balls and get engaged with dogs K1. Now the drive to the main shaft is direct from B via F1 and the splines. We have to give sufficient time for synchronization of speeds, otherwise, clash may result. For the second gear the members F1 and G1 are slid to the right so that finally the internal teeth on G1 are engaged with L1.then the drive to the main shaft will be from B via U1, U2, C, F1, and splines. For first gear, G2 and F2 are moved towards the right. In this case, the driver will be from B via U1, U3, D, F2, and splines to the main shaft.

For reverse, G2 and F2 are slid towards the right. In this case, the driver will be from B via, U1, U4, U5, D, F2 are splined to the main shaft.

Explain the construction and working of the synchromesh gearbox.

Construction Of Synchromesh Gearbox:

It consists of Clutch shaft, Layshaft, main shaft, and Synchromesh device. The gears on layshaft are rigidly fixed while gears on the main shaft are free to rotate on the main shaft with respect to the speed of layshaft. In this, a synchromesh device is used in between two main shaft gears to equalize the speed by friction after which these may be engaged smoothly.

A is the engine shaft having clutch gear B rotate at engine speed, gear B, C, D, E are on the main shaft and gear U1, U2, U3, and U4 are the gear on the layshaft. U5 is the intermediate gear. F1 and F2 are the synchromesh members free to slide on the main shaft which is spline internally.
G1 and G2 are the ring shape members having internal teeth which fit onto external teeth of member F1 and F2, K1 and K2, L1 and L2 are dog teeth on gear B, C, D, E, T1, and T2 are the ball supported by spring. S1 and S2 are the forks.

Synchromesh Gear box diagram — Synchromesh Gearbox diagram

Working of Synchromesh Gearbox :

To obtain low gear, member F2 moves towards the left which causes the friction contact between the cone-shaped surface of gear D and member F2. The friction or rubbing makes the speed equal then member G2 override to the left to engage with teeth K2 and low gear is obtained. Similarly, when member F2 slides toward the right, mesh with gear E, a reverse gear is obtained. Similarly for second gear the member F1 and G1 are slides to the right so that finally the internal teeth on G1 have engaged with L1 then the drive to the main shaft from gear B – U1 – U2 – gear C – G1 – F1 to spline. A direct gear is obtained by sliding member F1 toward left.

Application Synchronizer to avoid double-declutching

The main purpose of this unit is to synchronize the speed of the two gears before they are engaged. We know that in a running vehicle when we press the clutch & put the gear in a neutral position, till the gear are revolving. All the gear does not revolve at the same speeds & when we have to engage two gears running at different speeds by shift lever there will be some sound due to clashes of gears and very hard to engage and disengage the gears. To avoid said problems the synchromesh devices are used. Synchromesh devices are not fitted to all the gears. They are fitted only on the higher gears.
During synchronization, the synchronizer sleeve is moved towards selected gear pushing the block ring to the right, the ring contacts the shoulder of the driven gear and begins to synchronize the speed of the two parts. In this way the drive from layshaft gears to main shaft gear and then to the main shaft through synchronizer device.

Method of lubrication of synchromesh gearbox

Splash lubrication system

The splash lubricating system is simple to manufacture and implement in the synchromesh gearbox. Layshaft gear revolutions splash up lubricant from the sump to provide adequate lubrication to main shaft gears, Synchronizer, selector fork &, main shaft, lay shaft & clutch shaft Bearings components. The lubricant is thrown against the gravity inside the gearbox casing as droplets or fine mist.

Splash lubrication of synchromesh gear box

Different lubrication points of synchromesh gearbox:

1. The gearbox should always remain filled with lubricant. The gears are partially dipped in lubricating oil
2. It lubricates bearing of the gearbox
3. Selector mechanism is lubricated by thin machine oil/engine oil

Advantages Of Synchromesh Gearbox :

1) No need of double-declutching as in case of constant mesh gearbox.

2) Smooth engagement of higher gears due to the synchromesh device.

3) Less noisy as helical gears are used.

4) Less vibration.

Disadvantages of Synchromesh Gearbox :

Demerits of synchromesh gearbox:

1) Synchromesh is a finely machined element and hence is costly.

2) The service of gears and synchromesh device is difficult.

3) More space is required.

4) Use of synchromesh device for low-speed gears is uneconomical.

Application of synchromesh gearbox

Modern cars, Sport utility vehicles, for Higher gears in the gearbox

Difference Between Constant mesh gearbox and synchromesh gearbox

Sr. no.	Constant mesh gear box	Synchromesh gear box
1.	It has need of double-declutching.	No need of double-declutching as in case of constant mesh gearbox.
2.	The problem in the engagement of higher gears due to constant mesh devices.	Smooth engagement of higher gears due to synchromesh device.
3.	It is more noisy.	It is less noisy as helical gears are used.
4.	It has more vibration.	It has less vibration.

Checking of the synchromesh unit.

1. Check that all splines on the synchromesh hub are free from excessive wear.
2. Check that the engagement of dog teeth on the sliding sleeve and gear are free from chipping and burring.
3. Check that the synchronizer cones are not excessively worn or showing the effects of overheating.
4. Renew the springs and locking balls, if worn out.
5. Check synchronizer contact surfaces on the gears and cups for excessive wear; if burnt out contact surfaces are evident, gears or cups should be renewed.
6. Check blocker pin chamfer for excessive wear, Renew as necessary.

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